Handle error message edge case when finding yaml paths

[roomrys]

Description

I recently ran into an edge case where a SLEAP-created config file could not be found... the code then raises a FileNotFoundError with an informative error message - except it can't actually do this because the error message includes an unbound variable. Whoops.

    f"Cannot locate {shortname} config file at {desired_path} or {package_path}."
UnboundLocalError: local variable 'desired_path' referenced before assignment

This PR ropes that variable back into bounds 🤠

    f"Cannot locate {shortname} config file at {desired_path} or {package_path}."
FileNotFoundError: Cannot locate shortcuts.yaml config file at None or /home/talmolab/micromamba/envs/env_2/lib/python3.7/site-packages/sleap/config/shortcuts.yaml.

Types of changes

• Bugfix
• New feature
• Refactor / Code style update (no logical changes)
• Build / CI changes
• Documentation Update
• Other (explain)

Does this address any currently open issues?

[list open issues here]

Outside contributors checklist

• Review the guidelines for contributing to this repository
• Read and sign the CLA and add yourself to the authors list
• Make sure you are making a pull request against the develop branch (not main). Also you should start your branch off develop
• Add tests that prove your fix is effective or that your feature works
• Add necessary documentation (if appropriate)

Thank you for contributing to SLEAP!

❤️

[codecov]

Codecov Report

Merging #1456 (5af2a03) into develop (6858563) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1456   +/-   ##
========================================
  Coverage    73.05%   73.06%           
========================================
  Files          134      134           
  Lines        23856    23857    +1     
========================================
+ Hits         17428    17430    +2     
+ Misses        6428     6427    -1     

Files Changed	Coverage Δ
sleap/util.py	92.41% <100.00%> (+0.05%)	⬆️

... and 1 file with indirect coverage changes

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